Re: [PATCH] KVM: arm64: Don't let a guest overwrite HCRX_EL2.MCE2

[Oliver Upton <oupton@kernel.org>]

Hey,

On Mon, Oct 26, 2026 at 08:26:33AM +0000, Marc Zyngier wrote:
> FEAT_MOPS is fun. CPUs are allowed to implement any of two ways to
> perform the memory copy, and the hypervisor is in charge of hiding
> the difference between implementation when the vcpu migrates from
> one to another. This is unhelpfully signalled with an exception,
> which KVM diligently handles.
> 
> Today, we don't really make a good job of it in the sense that
> if L1 also has the trap bit set, we don't forward the trap as
> the architecture would require us to do.
> 
> But there is no good reason to forward this to the L1 guest if
> L2 generates the "I'm on the wrong implementation". KVM can fix
> it up in place and return to L2 directly, as if the physical CPU
> was able to implement both options.
> 
> Conversely, allowing L1 to clear HCRX_EL2.MCE2 is counter-productive,
> and violates the abstraction that KVM aims to give.

What abstraction? :-)

At least in the non-nested case we can blame the VMM since it's
responsible for scheduling vCPUs on the right pCPUs in a heterogeneous
system.

There is a subtlety here where if the VMM does the right thing with
sched affinities then a MOPS exception taken from L2 is definitely the
L1's fault and architecturally visible.

But oh well. We're obviously oriented towards presenting a homogeneous
set of vCPUs and anything else works by accident.

> Implement this behaviour by never allowing the guest's view of
> HCRX_EL2.MCE2 to make it into the HW, and document that KVM is in
> charge of the A/B handling.
> 
> Signed-off-by: Marc Zyngier <maz@kernel.org>

Reviewed-by: Oliver Upton <oupton@kernel.org>

Thanks,
Oliver